This invention relates to diagnostic and tissue stimulation devices such as implantable pacemakers, cardioverters and defibrillators, implantable monitoring devices and implantable drug dispensers, and more particularly to rate-responsive implantable pacemakers that vary their pacing rate as a function of the patient's metabolic demand for oxygenated blood.
Early pacemakers provided a fixed rate stimulation pulse generator that could be reset on demand by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operations, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
More recently, single and dual chamber pacemakers have been developed that respond to physiologic sensors which, with greater or lesser degrees of specificity, sense the body's need to deliver more or less oxygenated blood to the cardiovascular system. For example, rate responsive pacing systems have been developed and marketed which rely upon the patient's rate of respiration. Such pacemakers are described, for example, in U.S. Pat. Nos. 3,593,718 and 4,596,251 and have been commercialized by Biotec and Telectronics. These pacemakers use an impedance pneumograph for acquiring a respiration signal. More recently, it has been proposed to employ the variation and the amplitude of the peak-to-peak ECG signals as a rate control signal on the premise that the amplitude varies as a function of the patient's activity and/or respiration as disclosed in U.S. Pat. No. 4,757,815.
The impedance pneumograph measurement technique of the prior art involves the injection of a pulse or pulse burst of alternating current at subthreshold stimulation energy levels across a pair of electrodes and measuring voltage or current levels to derive an impedance measurement. The electrodes may be located one on either side of the chest, as in U.S. Pat. No. 4,596,251, issued to Plicchi, et al., both located in the heart as in U.S. Pat. No. 4,919,136, issued to Alt, or one electrode may be located in contact with the heart and one in the chest. U.S. Pat. No. 4,702,253 describes a system employing bipolar pacing electrodes. A constant voltage pulse train is injected into the tissue between one electrode and the pacemaker can and a measurement of the current taken between the other electrode in the heart and the pacemaker can. The impedance varies with exhalation and inhalation.
U.S. Pat. No. 4,805,621, issued to Heinze et al. suggests an approach to minimizing the effects of long term changes in overall tissue impedance on the accuracy of such systems. In the Heinze et al. device, the drive signal, as modulated by the tissue impedance is passed through a first low pass filter to strip off the drive signal frequency and produce a signal indicative of impedance variation over time, stated to correspond to respiratory activity. This impedance signal is then passed through a high pass filter to strip off the extremely low frequencies at which overall changes in tissue impedance are stated to occur.